Linear bi-directional incremental motors, and particulary such a motor having bi-directional clamps.
There exists an increasing need for compact linear motors for applications in precise positioning and movement of devices and components in diverse fields, such as optics, microscopy, avionics, spacecraft, medicine, analytical instruments, and others. A large number of devices designed to accomplish these tasks have been proposed; however, none is available that combines very small size, light weight, moderate speed, high precision and accuracy, with high force and power. This invention describes such a linear motor.
One such device is the Inchworm(copyright) actuator (U.S. Pat. No. 3,902,084 and 3,902,085), which uses a piezoelectric driver to generate incremental microscopic displacements of a drive shaft. This device relies on friction to move the drive shaft backward and forward between the movements. The available push force therefore is limited by the frictional force resulting from the force normal to the drive shaft, generated by the piezoelectric strain the drive shaft. The Inchworm actuator also has a limited range of displacement and a relatively slow speed.
Recently, a group at the University of California at Los Angeles has described and demonstrated a linear incremental actuator (MAD) in which MEMS fabrication methods are used to produce mechanical teeth with micrometer-scale dimensions. This actuator is capable of generating large push forces. See, Chen, Q., Kim, C. J., and Carman, G. P., xe2x80x9cMesoscale Actuator Device: Micro Interlocking Mechanism to Transfer Microloadxe2x80x9d, Sensors and Actuators, v. 73 (1999), pp. 30-36. The addition of the teeth to the Inchworm actuator has in fact dramatically improved the holding force of the device vs. the original Inchworm design.
U.S. Pat. No. 4,709,183 discloses a flat piezoelectric linear motor that functions similarly to the Inchworm actuator mentioned above in that the moving member is alternately clamped and unclamped in synchronism with the expansion and contraction of a piezoelectric actuator. The holding force relies on friction between the stationary and the moving smooth surface parts of the motor. Hence, this motor is also capable of only very limited push force restricted by the magnitude of the frictional force and the applied clamping force.
U.S. Pat. No. 5,319,257 describes a xe2x80x9cUnitaxial Constant Velocity Microactuatorxe2x80x9d. The device in accordance with the invention also uses piezoelectric clamps and actuators operating on a drive shaft passing through a bore in the housing of the motor. As in the above noted patents, this device also relies on friction between the surface of the drive shaft and the clamping units leading to the same limitations in push force.
One device, for which a patent application was filed by two of the inventors hereof, overcomes the force, power and speed limitations of prior art devices by replacing the previously used active-clamping devices with a passive ratchet mechanism. The result is that the force, speed, and power density, i.e., power per unit weight, are all significantly increased vs. the Inchworm and similar products of similar size and weight. Its limitation, however, due to its mechanical configuration, is in that a delay in motion occurs when the direction of motion is changed, especially under load.
The linear incremental piezoelectric motor subject of this invention adopts the traditional Inchworm concept mentioned above, while utilizing a multi-clamp mechanism to significantly increase the holding force and the friction between the surface of the base/rail unit and the multi-clamping unit. Thus, the resulting push force is considerably increased using this concept in the device, subject of this invention. The motor in the preferred embodiment uses MEMS (Micro Electro Mechanical Systems) technology in its critical components, facilitating a small size while achieving large push force and making it an ideal candidate for compact, high power linear density motors. The speed of this motor is similar to traditional Inchworm and like devices. As compared to the above-mentioned ratchet based mechanisms, said motor works well when changing, under load, the direction of the motion because the interdigitated multi-clamp generates a bi-directional holding force. The key design feature of the motor, compared with the traditional Inchworm or similar devices, is the multi-clamp mechanism, which many times increases the contact surface area, since said contact area is the sum of contact surfaces between the base/rail unit and the multi-clamps. Consequently it is possible to generate a much greater friction and push force.
The actuators employed in these motors, the so-called Induced Strain Actuators, include piezoelectric, ferroelectric, electrostrictive, magnetostrictive, and thermally expansive devices that have high stiffness. High stiffness is important to maintain the accuracy and repeatability of the motion increments under a load. Such actuator can produce expansions of up to 100 micrometers and are ideal for the motor subject of this invention.
The operational parameters of this motor make it suitable for many applications where the Inchworm actuators and similar prior art devices, because of their performance characteristics, could not be used; where much higher force and higher power density are essential, as is the small size of the unit, and the capability of repeatable and highly accurate micro- or macro-positioning. Furthermore, the design provides the possibility of scaling up or down the size of said motor and trading its size and weight for some of the operational parameters. The use of MEMS technology for the critical rail/multi-clamp components in this motor also facilitates lower manufacturing cost.
In accordance with their size, three classes of motors are identified that are based on the basic clamping concept disclosed herein: MEMS (1 to 100 xcexcm component dimensions), the preferred embodiment; meso class (100 xcexcm to 1 mm); and macro class ( greater than 1 mm)
Accordingly, one object of this invention is to provide an improved linear incremental motor that incorporates a multi-clamp mechanism that converts the reciprocal motion of the main actuator into incremental forward or reverse motion which may be via a motor shaft, and more specifically such a motor which can use a piezoelectric main actuator, can use an electrostrictive main actuator, can use a magnetostrictive main actuator, can use a ferromagnetic man actuator, and/or which can use a thermally expansive main actuator.
Still another object of this invention is to provide a motor as in said one object where the multi-clamps to have means for rigidly clamping on the rails during clamp state and incrementally moving on the rails during release state.
Another object of this invention is to provide a motor as in said one object using multiple micro-ridges to serve as rails, and more specifically where both side surfaces of said micro-ridges are contacted by a base of the motor during the clamping process, whereby the resulting friction force is accordingly increased by twice the number of the micro-ridges or rails on the base.
A further object of this invention is to provide an incremental motor which can be made using micromachining technology to fabricate the micro-ridges and interdigitated high force multi-clamp mechanisms, enabling them to be of microscopic dimensions, while enabling in a larger version of said motor to be made using conventional machining, wire electro-discharge machining, and other manufacturing processes.
Still further object of this invention is to provide an improved symmetrical version of said motor with two sets of rails or micro-ridges.
A further object of this invention is to provide an incremental motor as per said one object having a bridge-like structure with two joints between the main actuator and the multi-clamps.
Yet another object of this invention is to provide an improved motor as per said one object which can be configured in several models differing in size, weight, operating characteristics, and applications.